Silicon carbide is well known for its high strength, thermal stability, abrasion resistance, and its ability to withstand oxidizing and otherwise corrosive environments. As a result, silicon carbide has been employed in numerous applications, ranging from the exotic to the mundane, in which these properties are important. Such applications include various advanced ceramics in which the silicon carbide is fabricated by hot pressing, reaction sintering or by pressureless sintering.
Silicon carbide is also employed in various refractory tiles, bricks, blocks or shapes which can be used as kiln furniture, crucibles for molten metal, as linings in steel making furnaces, etc. The markets for these latter articles cannot support the cost and do not require the performance of fine-grained, pure silicon carbide. In these cases, the desired silicon carbide-containing articles can be produced by suspending particulate silicon carbide in a dissimilar bonding matrix.
In making such articles, silicon carbide powder can be combined with the bonding matrix per se, or the silicon carbide powder can be combined with chemical precursors and the bonding matrix generated in situ. Silicon carbide products produced by the latter process are referred to as "reaction-bonded silicon carbide" herein, and it is to this type of refractory product that this application is directed.
Reaction-bonded silicon carbide in which the bond phase is silicon nitride is well known. For example, U.S. Pat. No. 4,990,469 describes such materials and a process for preparing them from a mixture of silicon carbide particles, silicon, and a small amount of inorganic oxide. A green body fired under nitrogen at 1420.degree. C. exhibited density, porosity and bending strength of 2.83 g/cm.sup.3, 11% and about 27 Kpsi, respectively.
Silicon nitride-bonded silicon carbide refractories invariably contain a small amount of oxygen which is carried on the surfaces of the reactant powders and is present as silica in the refractory product. The free silica has a detrimental effect on high temperature properties, particularly the alkali resistance of silicon nitride-bonded silicon carbide.
This problems is addressed according to the instant invention by utilizing a sialon bond phase, e.g., .beta.'-sialon, for the fine-grained silicon carbide refractory. Sialons are compounds of silicon, aluminum, oxygen, and nitrogen which can be envisioned as derived from silicon nitride through the simultaneous substitution of silicon by aluminum and nitrogen by oxygen. This leads, in the case of .beta.'-sialon, to a solid solution which can range in composition as expressed by the formula: EQU Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z
where 0&lt;z&lt;.about.4.2. The case in which z=0 represents pure silicon nitride. Sialon formation provides a sink for the troublesome free silica mentioned above, the oxygen being incorporated into the sialon lattice.
FIG. 1 shows the Si--Al--O--N behavior diagram; the x and y axes extend from 0 to 100 equivalent %. The stable phases are indicated by the solid darkened areas. FIG. 1 shows a .beta.'-sialon solid solution range as the solid area extending from the Si.sub.3 N.sub.4 corner at the lower left toward the center of the diagram. There is also an O'-sialon solid solution range indicated near the y-axis. O'-sialons derive from silicon oxynitride, Si.sub.2 N.sub.2 O, very much as .beta.'-sialons derive from .beta.-silicon nitride, Si.sub.3 N.sub.4, i.e., through partial substitution of Al and O atoms for Si and N atoms, respectively. Although the .beta.'-sialon range of compositions is very small, the O'-sialon range is even smaller. According to FIG. 1, the highest substitution occurs at about 7 equivalent % Al, represented by the atomic formula: EQU Si.sub.1.8 Al.sub.0.2 N.sub.1.8 O.sub.1.2
.beta.'-Sialons, derived from .beta.-silicon nitride, have physical and mechanical properties similar to those of silicon nitride. Their chemical properties are intermediate between those of silicon nitride and alumina and seem to depend to some extent on the value of the z parameter (also called "substitution number"). Chemical characteristics of .beta.'-sialons, such as resistance to corrosion, are believed to approach those of alumina as the z value increases. In addition, the fact the sialons are solid solutions leads to their lower vapor pressure and decreased tendency to decomposition at high temperatures relative to pure silicon nitride. For these reasons, sialon may be preferred over silicon nitride as a bond phase for silicon carbide refractories in those applications where resistance to certain corrosive environments and thermodynamic stability at high temperatures are of primary importance. Sialon bonding offers the potential for, among other things, improved alkali resistance, creep resistance, and resistance to molten metals as compared to silicon nitride. In addition, if the substitution number z is not too large, the oxidation resistance of the sialon-bonded silicon carbide will remain very good.
A reaction-bonded silicon carbide refractory having a Si.sub.3 N.sub.4 bond phase modified by the addition of sialon formers is described in U.S. Pat. No. 4,578,363. Reaction-bonded silicon carbide was produced by firing a green compact containing a mixture of silicon carbide powders, as well as silicon and aluminum powders, under nitrogen at 1420.degree. C. The only oxygen introduced was the thin layer of silica believed to be present on the silicon and SiC powders. The majority of the bond phase in the resultant product was nonetheless .beta.'-sialon. The density, porosity and bending strength of the article were about 2.6 g/cm.sup.3, 14% and about 5 Kpsi, respectively. The resistance of the product to attack by molten alkali was shown to be superior to that of a Si.sub.3 N.sub.4 -bonded silicon carbide article, but the resistance to steam oxidation was not improved. The relatively large particle sizes of some of the components precluded the use of the slip casting process to make the reaction-bonded products.
Sintered refractories composed of .beta.'-sialon, either added per se or produced in situ, are described in U.S. Pat. No. 4,243,621. It is disclosed that a green body containing .beta.'-sialon precursors can also include an aggregate of medium to coarse-grained (i.e., .ltoreq.250 micrometer) alumina, silicon nitride, silicon carbide, .beta.'-sialon, zirconia, or zircon. For example, when SiC was employed as an aggregate constituting about 65 wt % of the product, the fired refractory resulting from a pressed compact exhibited a density of about 2.6 g/cm.sup.3, 19% porosity, and a bending strength of about 5 Kpsi. After refiring the product under nitrogen, these numbers became 2.7 g/cm.sup.3, 10% and 5.4 Kpsi, respectively.
U.S. Pat. No. 4,506,021 describes O'-sialon ceramic products, and Jack, K. H., J. Mater. Sci., 11, 1135 (1976), is a review of sialons and related nitrogen ceramics.